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Heartworms in dogs

Today we conclude our series on parasites in dogs by moving away from worms that affect the intestinal tract of dogs, to a worm that—as the name suggests—lives in the hearts of dogs, but is not at all loved by the dog.

A heartworm (Dirofilaria immitis) is a parasitic nematode (roundworm) that is transmitted from host to host through the bites of mosquitoes. The life cycle of the heartworm requires two hosts for completion: a primary host which is the dog, and an intermediate host which is the mosquito.

A mosquito carrying infective heartworm larvae bites the dog and the larvae are injected into the dog’s bloodstream. These tiny larvae migrate through the dog’s body harming arteries and organs as they go, until they reach the heart and blood vessels of the lungs (pulmonary arterial system)—a process taking approximately two months.

Over the next four to five months, the larvae increase in size, maturing into adults, which can grow to be about 12 inches in length and can live for five to seven years.

A dog must carry at least two heartworms—a male and a female—for reproduction to occur. Females produce live babies or immature heartworms called “microfilariae,” which are shed into the dog’s bloodstream but are incapable of causing heartworm: they must first pass through a mosquito. The microfilariae can circulate in the bloodstream for as long as two years waiting for the intermediate host. When a mosquito bites the heartworm-infected dog, the microfilariae are taken into the mosquito’s body. The microfilariae transform into infective heartworm larvae over a two-week period inside of the insect and when the mosquito bites another dog the process starts again, thereby continuing the parasite’s life cycle and spreading the disease to the next host.

Dogs generally show no symptoms of the disease during the first six months of infection until the larvae mature into adults and congregate inside the heart and pulmonary arteries. Even then, dogs with only a light infection who live a fairly sedentary lifestyle may show little or no sign of infection. As the number of heartworms in the dog accumulates gradually over months and years, symptoms become more noticeable and include laboured breathing, coughing, vomiting, weight-loss, listlessness, reluctance to move or exercise, fatigue after moderate exercise, reduced appetite, fainting spells, high blood pressure, rapid heartbeat, coughing up blood, and finally congestive heart failure.

Heartworm disease is diagnosed by examination, radiographs (x-ray images of the heart and lungs) or ultrasound, and a veterinarian-administered blood test. The blood test is positive for heartworm if a heartworm substance called an antigen or microfilariae are detected.

Although treatment of heartworm has a high success rate in all but the most advanced cases, this cure consists of a series of injections called adulticides into the dogs’ muscle and usually requires hospitalisation. Several weeks of exercise restriction are also needed to give the body sufficient time to absorb the dead worms without ill effect. Otherwise, when the dog is under exertion, dead worms may break loose and travel to the lungs, potentially causing respiratory failure and death. In some cases, a surgical procedure may be necessary to remove adult worms from the right heart and pulmonary artery by way of the jugular vein. This procedure is recommended if the infestation consists of a high number of adult worms.

Therefore, as always, disease prevention is a much better and safer option. The good news is that heartworm infection is easy, safe and inexpensive to prevent. There are a variety of options for preventing heartworm infection in dogs, including veterinary prescribed monthly pills and chewable tablets, monthly topical treatments and six-month or annual injectable products. Talk to your registered veterinarian about how to best protect your dog against this dangerous disease.

The heartworm is of negligible public health risk, because it is unusual for humans to become infected. In the rare cases where heartworms have infected people, the worm cannot complete its life cycle. Instead, the heartworm migrates to the human lung, dies, and forms a nodule that looks like a tumour.